Generally, the furnace or combustion area of a steam generator is enclosed either solely by vertical boiler tubing or it is enclosed by a combination of spiral and vertical boiler tubing. As is well known, vertical boiler tubing is generally self-supporting while spiral boiler tubing requires extensive exterior support. This is because vertical boiler tubing must generally only withstand tensile or compressive forces in one direction along its vertical "Y" axis (in addition to its internal pressure forces) and thus need only be supported against buckling. In contrast, spiral boiler tubing must carry its load in three directions (along the "X", "Y", and "Z" axis) as it ascends in a spiral manner around the furnace or combustion area. Such spiral boiler tubing is thus subject to tension, compression, shear, moment, torsion, and hoop forces in addition to its internal pressure forces.
Despite such structural drawbacks, spiral boiler tubing has an operational advantage over vertical boiler tubing since it provides parallel flow paths around the combustion chamber enabling each tube to pass through very similar zones of heat input intensities. This enables the fluid contained within each spiral boiler tube to absorb nearly the same amount of heat from the different heat zones within the combustion chamber. In contrast, vertical boiler tubing provides vertically parallel flow paths with the fluid contained therein absorbing considerably different amounts of heat from fewer different heat zones within the combustion chamber. Consequently, the fluid flow heat balance between adjacent tubes, or from tube to tube, is inherently better with spiral tubing than with vertical tubing, especially at subcritical pressures. This is especially advantageous for once-through steam generators.
In the past, spiral boiler tubing was held or supported in place by using plates that were welded to the outside of the tubing. These plates themselves were then secured to structural members that, in turn, transferred the load to other supporting structure located either above or below the spiral tubing, such loads oftentimes included some of the burner and windbox loads as well. When the spiral tubing was supported from above, such loads were secured to either the upper vertical boiler tubing or to upper boiler support steel. When the spiral tubing was supported from below, such loads were transmitted to the foundation via adjacent structural members and spring type supports.
One method of supporting the spiral tubing from upper vertical tubing involves a plurality of plate-type straps. These straps would be continuously welded to the outside of the spiral tubing in a vertical direction. Generally, these straps would extend across the spiral tubing section of the furnace enclosure from the lower furnace hopper to the upper vertical tubing. Upon reaching the upper vertical tubing, the straps would be paddled with multiple fingers in an attempt to evenly distribute such loading amongst the vertical tubing. In this fashion, the loading of the underneath spiral tubing would be transferred to the upper vertical tubing.
As is well known, it is quite common for steam generators to operate at variable furnace pressures or for them to frequently cycle between being on-line and off-line. While these generators can be designed for such variations, such designs can accommodate only a finite number of such cycles before they fail. Consequently, securing any type of welded plate to the outside of the furnace enclosure will impact upon the designed life span of the generator by adding to the already existing stresses and strains of the spiral tubing. Such additional positive and negative thermal stresses will cause or contribute to increased fatigue damage, more pressure part failures, reduced life expectancy of the tubing, enhanced potential for outages, and greater need for corrective maintenance and repair.
It is thus an object of this invention to provide a means of supporting spiral tubing that does not rely upon the use of external support plates welded or otherwise secured to the tubing. A further object of this invention is to provide a means of more evenly dispersing the loading of the spiral tubing onto the upper vertical tubing if this is desired. Another object of this invention is to provide a means of concentrating the loading of the spiral tubing into a few select locations for subsequent transfer elsewhere if so desired. Yet another object of this invention is to provide a means of support that is economical to both construct and install. Still another object of this invention is to provide a means of supporting the spiral tubing that will not cause or create additional temperature differentials within the tubing. Another object of this invention is to provide a means of supporting the spiral tubing without compromising the designed life expectancy of the generator, its tubing, or its various components. Yet another object of this invention is to support such spiral tubing without increasing the occurrence of fatigue damage. These and other objects and advantages will become obvious upon further investigation.